The present invention relates to a radiation image capturing apparatus, particularly to a radiation image capturing apparatus wherein a radiation image capturing apparatus performs radiation image capturing operations by detecting irradiation by itself.
There has been development of various types of radiation image capturing apparatuses including a so-called direct type radiation image capturing apparatus that generates an electric charge through a detection element in response to the dosage of applied radiation such as X-rays and converts the electric charge into an electric signal, and a so-called indirect radiation image capturing apparatus that uses a scintillator etc. to convert the applied radiation into electromagnetic waves having other wavelengths such as visible light, then generates an electric charge through a photoelectric conversion element such as a photodiode in response to the energy of the electromagnetic wave having been converted and applied, and converts the electric change into an electric signal (i.e., image data). In the following description of the embodiments of the present invention, the detection element in the direct type radiation image capturing apparatus and the photoelectric conversion element in the indirect radiation image capturing apparatus will be collectively referred to as a radiation detection element.
This type of radiation image capturing apparatus is known under the name of FPD (Flat Panel Detector). In the conventional art, this radiation image capturing apparatus has been designed to be formed integrally with a support base (or bucky device) (refer to the Unexamined Japanese Patent Application Publication No. Hei 9(1997)-73144, for example). In recent years, there has been development of portable radiation image capturing apparatuses wherein a radiation detection element and others are incorporated in a housing for easy transportation. These portable radiation image capturing apparatuses have been put into practical use (refer to Unexamined Japanese Patent Application Publication No. 2006-058124, and Unexamined Japanese Patent Application Publication No. Hei 6(1994)-342099).
In the aforementioned radiation image capturing apparatus, a plurality of radiation detection elements 7 are normally arranged in a two-dimensional array (matrix) on a detecting section P, and each radiation detection element 7 is connected with the switch unit formed of a thin film transistor (hereinafter referred to as “TFT”) 8, as shown in FIG. 6 to be described later. The on-voltage or off-voltage is applied to the scanning line 5 from the gate driver 15b of the scanning drive unit 15 so that the on/off operation of each TFT 8 is performed. Then the electric charge is stored into each radiation detection element 7, or is discharged from each radiation detection element 7 to each signal line 6.
Incidentally, a radiation image is formed by application of radiation to the radiation image capturing apparatus from the radiation source of a radiation generation device through a subject. For example, if a radiation image capturing apparatus and radiation generation device are produced by the same manufacturer, it is possible to design such a structure that an image can be captured while a signal or information is exchanged between the radiation image capturing apparatus and radiation generation device.
However, if a radiation image capturing apparatus and radiation generation device are produced by different manufacturers, a signal or information may not be exchanged between the radiation image capturing apparatus and radiation generation device in some cases. In this case, the radiation image capturing apparatus is required to make sure that radiation has been applied by itself.
To solve this problem, in recent years, it has been known that development of various radiation image capturing apparatuses capable of self-detection of the application of radiation, independently of the aforementioned interface configured between the radiation image capturing apparatus and radiation generation device.
For example, according to the Specification of the U.S. Pat. No. 7,211,803 and the Unexamined Japanese Patent Application Publication No. 2009-219538, when exposure of the radiation image capturing apparatus to radiation has started, and electric charge has been generated inside each radiation detection element 7, electric charge flows from each radiation detection element 7 to the bias line 9 (refer to FIG. 6 to be described later) connected to each radiation detection element 7, with the result that there is an increase in the volume of current running through the bias line 9. It is proposed that to utilize this phenomenon effectively, the bias line 9 is provided with a current detection unit to detect the value of the current flowing through the bias line 9 and that thus, the start of irradiation is detected based on this current value.
According to the research made by the present inventors, however, it has been found out that since the aforementioned technique uses a bias line 9 connected to the electrode of each radiation detection element 7, noise generated by the current detection unit is transmitted to each radiation detection element 7 through the bias line 9, and is superimposed on the image data D read out of the radiation detection element 7 in some cases and that solution to the problem is not easy.
In the meantime, after extended research on an alternative method that enables the start of irradiation to be detected by the radiation image capturing apparatus, the present inventors have found out several techniques that enable the radiation image capturing apparatus to detect the start of irradiation appropriately by itself.
Incidentally, as will be described later, in one of the procedures of detecting the start of irradiation, which have been found out by the present inventors, off-voltage is applied to each scanning line 5 from the scanning drive unit 15 prior to radiation image capturing. When each TFT 8 has been turned off, the reading circuit 17 is allowed to perform read-out operation. Thus, the leak data “d leak” is read out in such a way that the electric charge “q” (see FIG. 10 to be described later) having leaked out of the radiation detection element 7 through the TFT 8 is converted into the leak data “d leak”.
When the radiation image capturing apparatus has been irradiated, the value of the leak data “d leak” to be read is increased. This is employed to detect the start of irradiation of the radiation image capturing apparatus, based on the value of the leak data “d leak” having been read out. In this case, if each TFT 8 is kept turned off, dark electric charge will be accumulated in each radiation detection element 7. Thus, as will be described later, the step of reading the leak data “d leak” and the step of resetting each radiation detection element 7 are repeated on an alternate basis.
In another procedure of detecting the start of irradiation found out by the present inventors, before the radiation image capturing operation, on-voltage is sequentially applied to each of lines L1 through Lx of the scanning line 5 from the scanning drive unit 15 so that image data “d” is read. In the following description, the image data for detecting the start of irradiation read out for detecting the start of irradiation prior to the above-mentioned radiation image capturing operation is referred to as image data “d”, as distinguished from the image data D as a main image immediately after image capturing operation.
In this case as well, when the radiation image capturing apparatus is irradiated, there is a rise in the value of image data “d” to be read. This is utilized again to detect the start of irradiation of the radiation image capturing apparatus, based on the value of the image data “d” having been read out.
This structure is known to permit the radiation image capturing apparatus by itself to accurately detect irradiation based on values of the leak data “d leak” or image data “d” having been read out, even if an interface cannot be formed between the radiation image capturing apparatus and radiation generation device.
In the radiation image capturing apparatus, when the start of irradiation of the radiation image capturing apparatus has been detected in conformity to the above-mentioned procedure, off-voltage is applied to each scanning line 5 from the scanning drive unit 15 and each TFT 8 is turned off, and then the state transfers to the state of electric charge accumulation where the electric charge generated by irradiation is accumulated in each radiation detection element 7. After that, on-voltage is sequentially applied to each scanning line 5 from the scanning drive unit 15, thereby starting the step of reading the image data D as the main image from each radiation detection element 7.
In this case, in the state of electric charge accumulation where each TFT 8 is kept turned off, so-called dark electric charge is accumulated in each radiation detection element 7 at all times due to the thermal excitation caused by the heat (temperature) of the radiation detection element 7 itself. The offset resulting from this dark electric charge is superimposed on the image data D to be read out.
Thus, the step of reading the offset data O where the offset resulting from the dark electric charge is read out as offset data O is executed after the image data D as the main image has been read out, in many cases. Then the true image data D* is calculated by subtracting the offset data O having been read out from the image data D read out in the subsequent image processing step. This procedure ensures that the true image data D* is purely the data resulting from the electric charge generated by irradiation, without including the offset resulting from dark electric charge.
Incidentally, when the structure is so configured as to perform the step of reading the above-mentioned offset data O, various conditions are set up in such a way that the offset data O will be the value to accurately cancel the offset resulting from the dark electric charge superimposed on the image data D as the main image in the above-mentioned step of subtraction.
To be more specific, for example, after the image data D as the main image has been read, the image data D left without having been read out will remain in each radiation detection element 7. To solve this problem, for example, the step of resetting each radiation detection element 7 is repeated to remove the unread portion of the data subsequent to the step of reading the image data D. After that, the offset data O is read out.
However, when the above-mentioned new procedure of detecting the start of irradiation found out by the present inventors is adopted, it has been revealed that there may be a failure of offsetting, in some cases, between the offset resulting from the dark electric charge superimposed on the image data D as the main image and the offset data O, if the above-mentioned procedure of reading the offset data O that has been regarded as natural.
This will be described more specifically. The image capturing operation should be performed by irradiation of the radiation image capturing apparatus when a subject is present between the radiation source and radiation image capturing apparatus. However, for the sake of clarity, radiation is assumed to be applied uniformly to each radiation detection element 7 of the radiation image capturing apparatus without any subject.
To be more specific, on-voltage is sequentially applied to each of lines L1 through Lx of the scanning line 5 from the gate driver 15b of the scanning drive unit 15, without radiation applied to the radiation image capturing apparatus, as illustrated in FIG. 27. Then the image data “d” for detecting the start of irradiation is read out of each radiation detection element 7. Radiation is then uniformly applied to the radiation image capturing apparatus. The start of irradiation is assumed to be detected based on the image data “d” read out when the on-voltage is applied to the line Ln of the scanning line 5.
In this case, application of the on-voltage to each scanning line 5 from the gate driver 15b is suspended at that moment. The off-voltage is applied to each scanning line 5 so that there is a transfer to the state of electric charge accumulation. In this state of electric charge accumulation, the electric charge generated in each radiation detection element 7 by irradiation is accumulated in each radiation detection element 7.
After the state of electric charge accumulation has been maintained for a prescribed period of time, application of the on-voltage is resumed, for example, from the line Ln+1 of the scanning line 5. The on-voltage is sequentially applied to the lines Ln+1 through Lx and L1 through Ln of the scanning line 5, and the image data D as the main image is read out of each radiation detection element 7 connected to each scanning line 5.
To remove the image data which is left without having been read out and which still remains in each radiation detection element 7, on-voltage is sequentially applied to the lines Ln+1 through Lx and L1 through Ln of the scanning line 5 so that the step of resetting each radiation detection element 7 is repeated. After that, on-voltage is sequentially applied to the lines Ln+1 through Lx and L1 through Ln of the scanning line 5 to read out the image data “d” for detecting the start of irradiation. The equivalent step of resetting the each radiation detection element 7 can be executed, instead of reading the image data “d” for detecting the start of irradiation.
After that, the radiation image capturing apparatus is left to stand (wherein radiation is not applied in this case) for the same time duration as that for the above-mentioned state of electric charge accumulation while on-voltage is applied to each of lines L1 through Lx of the scanning line 5. Then, application of on-voltage is resumed from the line Ln+1 of the scanning line 5, so that on-voltage is sequentially applied to the lines Ln+1 through Lx and L1 through Ln of the scanning line 5. The offset data O is read out from each radiation detection element 7 connected to each scanning line 5.
If the above-mentioned test is conducted, the offset due to the dark electric charge superimposed to the image data D read out as the main image, and the offset data O having been read out thereafter should reach the same value for each radiation detection element 7. Accordingly, the true image data D* is calculated by subtraction of the offset data O from the image data D and this should provide offsetting between the offset superimposed on the image data D and offset data O. Thus, the true image data D* should be the same value for each radiation detection element 7.
In actual practice, however, as shown in FIG. 28, it has been revealed that there is a level difference in the true image data D* between the line Ln of the scanning line 5 which has detected the start of irradiation and the line Ln+1 of the scanning line 5 which has started reading of the image data D first. In FIG. 28, the magnitude of the true image data D* is represented in terms of contrast ratio. For easy viewing of the figure, the difference in the magnitude (contrast ratio) is emphasized.
The level difference in the true image data D* also appears when radiographic imaging operation is performed using a subject. As shown above, if the level difference occurs to the true image data D*, a streak will appear on the image portion corresponding to the level difference on the true image data D*, for example, when the radiation image captured by the radiation image capturing apparatus is used for diagnosis in medical treatment. This will cause a viewing difficulty. Further, if this streaked image portion and the lesion of the patient are overlapped with each other on the radiation image, viewing difficulty of the lesion will arise, with the result that the lesion may be overlooked.